Being so close to sonic velocity small factors might make it go supersonic, old thunderbolt has this problem, it's so close that some make the crack and some don't, and I've heard that supersonic bullets fly differently than subs, which is why subs are supposed to be more accurate...

When any object (airplane or bullet) approaches transonic speed it passes through it's own "Precursor" wave, (think the bow wave from a boat only in air) and experiences buffeting from the shockwave as it passes through. Subsonic rounds never experience this buffeting and are usually more accurate because of it.

Sealed suppressors are commonly cleaned by making a "tank" out of a capped off length of plastic sewer pipe and filling with a solvent. Depending on solvent and paint used, it might strip the paint off. Serial numbers are engraved in the metal, paint coverage would not affect the number.